An active matrix substrate for use in liquid crystal display devices, or the like, includes a switching element, such as a thin film transistor (hereinafter, “TFT”), in each pixel. Examples of such a switching element which have been conventionally used in various applications include a TFT which includes an amorphous silicon film as the active layer (hereinafter, “amorphous silicon TFT”) and a TFT which includes a polycrystalline silicon film as the active layer (hereinafter, “polycrystalline silicon TFT”).
In recent years, using an oxide semiconductor as the material of the active layer of a TFT, instead of amorphous silicon or polycrystalline silicon, has been proposed. Such a TFT is referred to as “oxide semiconductor TFT”. The oxide semiconductor has a higher mobility than the amorphous silicon. Therefore, the oxide semiconductor TFT is capable of operating at a higher speed than the amorphous silicon TFT. The oxide semiconductor film is manufactured through a simpler process than the polycrystalline silicon film and is therefore applicable to devices which require a large area. An active matrix substrate which includes an oxide semiconductor TFT (hereinafter, “TFT substrate”) is disclosed in, for example, Patent Document 1.
For example, Patent Document 2 discloses integrally forming a semiconductor layer which serves as the active layer of a TFT and an electrically-conductive layer, such as a pixel electrode, by reducing the resistance in part of the oxide semiconductor film. Patent Document 2 discloses that the above-described resistance reducing process enables to manufacture a TFT substrate which includes an oxide semiconductor TFT at a lower cost.
In the case where a conventional TFT substrate disclosed in, for example, Patent Document 1 or 2 is used in a liquid crystal display device, storage capacitance is provided in the TFT substrate when necessary. The storage capacitance is formed by a storage capacitance electrode (or storage capacitance wire), a pixel electrode, and a dielectric layer interposed therebetween. The storage capacitance electrode is formed by, for example, an electrically-conductive film which is the same as a gate wire.